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Giants in the Gulf

Whale sharks are closer than you think.

By Melissa Gaskill

Rays of sunlight spear gem-blue waters where the southern Gulf of Mexico and Caribbean mingle just off the Yucatán Peninsula’s northern tip. In late summer, tiny plankton cloud the usually clear sea like sugar in a glass of tea. A ghostly form covered in white spots suddenly looms out of the cloudiness, its open mouth a dark cavern in a wide, flat head. The whale shark measures some 28 feet long, roughly the length of a nearby boat. Oblivious to snorkelers less than 10 feet away, it motors on, swinging a massive tail. While that tail could deliver quite a smack to those snorkelers, these sharks have only tiny teeth.

The world’s largest fish, Rhincodon typus can grow longer than 50 feet. Even though it’s big, it eats primarily plankton — the small animals, plants and microbes that drift in ocean currents. Feeding whale sharks swim slowly, sucking in plankton-rich seawater and passing it through a filtering structure in their gills. While often seen feeding at the surface, they can dive 4,500 feet deep and eat larger organisms such as small squid.
Common sights around Australia, India and the Philippines, these generally solitary creatures also congregate off the southern shore of Belize in spring to feed on fish spawn.

Then, each summer, they arrive off Mexico’s Yucatán Peninsula, where a current roaring up the continental shelf drags nutrient-rich deep waters with it. The resulting plankton bloom rings the whale shark dinner bell, and the feast lasts until mid-September.

But until recently, not many people knew that the gentle giants also venture farther into the Gulf of Mexico. Eric Hoffmayer, a researcher with the University of Southern Mississippi’s Gulf Coast Research Laboratory, came across a pair in the northern Gulf in 2002. Talking with deep-sea fishermen, he learned they had been spotting whale sharks for years, often in schools of up to 100.

“There was nothing like that out there anywhere in the world. It was unheard of to have a group of more than 15,” says Hoffmayer, who suspects the sharks come to the northern Gulf for an all-you-can-eat buffet when bonito, skipjack and tuna gather to spawn. He established a sighting database and distributed posters to dive shops and fishing outfitters, asking people to report whale sharks in the Gulf of Mexico. From 2002 to 2007, 60 sightings were reported; in 2008, 75. All occurred between April and November, with 86 percent between June and October. Most centered around the Mississippi River, perhaps because of the supply of nutrients the river carries.

Then, last summer, scientists collected the first proof of the connection between populations at the Mesoamerican Reef, which runs from Honduras up to the Yucatán, and those of the northern Gulf. According to Rachel Graham, associate conservation scientist with the Wildlife Conservation Society’s ocean giants program, scientists retrieved data in August indicating that a whale shark tagged near Isla Holbox, Mexico, had triggered a detector at Bright Bank the previous November. That reef lies 14 miles east of the Flower Garden Banks National Marine Sanctuary, about 100 miles from the coasts of Texas and Louisiana. In September 2009, Graham also tracked tagged individuals “doodling around” off South Padre Island, where divers often see them circling rigs.

This solid evidence of movement between the Mesoamerican and Gulf reef populations will help efforts to foster regional accords for conservation and management of a variety of marine species, Graham says. With the Flower Garden Banks management plan currently under review, the National Oceanic and Atmospheric Administration is considering requests to add an additional nine banks and reefs to the sanctuary, including the one visited by the tagged whale shark. Marine Meganet, a joint project involving Graham, Hoffmayer and Emma Hickerson, a researcher at the Flower Garden Banks management office in Galveston, plans to tag additional whale sharks and expand the number and range of acoustic receivers to gather additional data on the whale sharks’ movements.

Tagged individuals feeding in Belize’s spring snapper spawn have been tracked moving from there to Honduras, according to Julianne Stockbridge, marine program manager for the Nature Conservancy in Belize, while others headed to the Yucatán. Stockbridge has no tagging evidence as yet, but is sure others end up in the Gulf of Mexico. Marine biologist Kenneth Johnson, owner of EcoColors outfitter in the Yucatán, says tags he helped place on more than 600 whale sharks there show that some move from the Caribbean to the feeding area off Cancún and back. Others go farther north into the Gulf, while some go south. In other words, he sees no defined pattern of movement. Further, he says, the large Yucatán aggregations include only a few tagged sharks, which tells him that the same individuals don’t necessarily hit the plankton buffet every year.

Tagging evidence to date, and the knowledge that whale sharks frequent the northern Gulf, southern Gulf and Caribbean around the same time — June to September — suggest that individuals move freely between these areas, joining different aggregations to feed based on opportunity. The sharks can easily swim 10 kilometers daily, Graham says.

Scientists bearing tags and divers seeking thrills haven’t been the only ones noting the presence of these behemoths. Yucatán fishermen noticed the whale shark aggregations about seven years ago, Johnson says, and started taking tourists to see the impressive sight. Concerned for the animals’ welfare, the World Wildlife Fund asked Johnson to organize tourism activity and protect the sharks. He worked with Graham, who had helped develop rules in Belize, and others to establish rules to minimize tourism’s impact on the animals in Mexico.

“The main rule is no touching,” he says. “Not because that could hurt the whale sharks, but to avoid disturbing or affecting them.” Persuading the curious to avoid interfering with the docile creatures is a challenge, says Hickerson, who has received photographs of people riding on whale sharks. Candid snorkelers and divers report that when they move in front of them, the animals often dive. This, of course, interrupts their feeding.

Stockbridge hopes to document the effects of tourism in the Belize whale shark zone, but doesn’t have enough data yet to draw conclusions. Trained guides there are primarily locals, she says, and have a vested interest in following the guidelines and helping maintain the integrity of the shark population to protect their livelihood.

Well-managed tourism also can be beneficial in other ways. “It reconnects people with nature, and gives them the opportunity to interact with a large animal in a way they aren’t able to anywhere else on earth,” Graham says. “Imagine being five feet away from a full-grown elephant, rhino or large lion. You can’t do it. Whale sharks provide this absolutely phenomenal experience, and for many people it is life-changing. Tourism can definitely be a good thing.”

The fact that tourism occurs at key feeding sites presents particular challenges, though. “What happens if, every time you go to a restaurant, people mob you, and you can’t even eat?” Graham asks. “You’re going to stop going. In Belize, we’ve seen change in the behavior of the spawning fish and then, of course, in the whale sharks themselves. We have to be very careful that tourism doesn’t overwhelm these animals. If we start making it difficult for them to come to these key feeding sites, we affect their fitness and ultimately their survival.”

The International Union for Conservation of Nature’s Red List of Threatened Species ranks whale sharks as vulnerable, a step below endangered, with the population currently decreasing. Top threats include the sharks’ high value in international trade and targeting by directed fisheries. Their low abundance and migratory nature make them particularly vulnerable to exploitation, and whale sharks have suffered from high human predation (as have most types of sharks). Currently, Mexico, the Philippines, Australia and the United States, among others, protect the sharks from fishing, and Graham thinks populations in the Gulf of Mexico and Caribbean remain in pretty good shape.

Other threats include ocean pollution and ship traffic. Last summer, Graham reports, a large aggregation of whale sharks occurred right in the middle of a shipping lane.

“I am very concerned, because shipping has been a major threat to the sharks,” Graham says. “It will be particularly bad if we get ships plowing through massive aggregations.” Documentation of the large aggregation can serve as a basis for discussions with the shipping industry, though, she says. “We’ll engage them to find mitigating measures to protect whale sharks. My goal is to see whether we can know enough about the aggregations that we can ask shipping companies to please deviate slightly to avoid the key areas during this particular time.”

If not, these amazing speckled giants could end up as pelagic road kill. Scientists and snorkelers everywhere hope that won’t happen.

SPOTTING WHALE SHARKS

Sharp-eyed boaters, fishermen and divers can report whale shark sightings and submit photographs two ways. Ecocean Whale Shark Photo-ID Library collects photographs of individual whale sharks. New photos are compared against already identified sharks using a pattern-recognition software originally developed by NASA to analyze data from the Hubble Space Telescope. The Whale Shark Project, organized by Project AWARE, an international nonprofit involving the scuba diving community, collaborates with Ecocean, encouraging divers to submit images for the database — and to follow guidelines for whale shark interaction.

Sightings in the northern Gulf of Mexico can be reported to the Gulf Coast Research Laboratory at the University of Southern Mississippi.